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CORROSION RESISTANT ALLOY
As the world-wide search is turning to deeper reservoirs an increasing number of situations are being encountered where corrosive production environments are present. In many of these cases often significant amounts of hydrogen sulphide, carbon dioxide and brine are present with oil and gas production. These crudes show, therefore a high corrosivity with respect to general corrosion and stress corrosion cracking by sulphides ( SSCC ), by chloride (CSCC) or their combined action.
In addition, other factors such high pressures and temperatures can complicate the material selection process. In fact, the mechanical requirements for material used for production equipment increase with well depth because of the greater hangoff loads and pressure; while the elevated temperatures have detrimental influence on mechanical properties. Under these circumstances CRA materials may offer a valid alternative to conventional methods of corrosion control. Based on that the use of corrosion-resistant alloy in oil field has substantially increased during the last years.
With the term CRA is intended a metal that achieves a high corrosion resistance by means of alloying. A variety of CRA materials are now available for tubing. Table 1 shows some of the commonly use for oil and gas production application. Depending on the environment the CRA choice could range from AISI 420 ( 13% Chrome) for CO2 service to titanium alloys for very severe applications. The first topic of discussion will be manufacturing process, with some discussion on how the different processes can influence the final product performances.
In addition, other factors such high pressures and temperatures can complicate the material selection process. In fact, the mechanical requirements for material used for production equipment increase with well depth because of the greater hangoff loads and pressure; while the elevated temperatures have detrimental influence on mechanical properties. Under these circumstances CRA materials may offer a valid alternative to conventional methods of corrosion control. Based on that the use of corrosion-resistant alloy in oil field has substantially increased during the last years.
With the term CRA is intended a metal that achieves a high corrosion resistance by means of alloying. A variety of CRA materials are now available for tubing. Table 1 shows some of the commonly use for oil and gas production application. Depending on the environment the CRA choice could range from AISI 420 ( 13% Chrome) for CO2 service to titanium alloys for very severe applications. The first topic of discussion will be manufacturing process, with some discussion on how the different processes can influence the final product performances.
Corrosion Resistant Alloys (CRAs) are essential for providing long term resistance to corrosion for many components exposed to oil and gas production environments. Components include downhole tubing and safety critical elements, wellhead and Xmas tree components and valves, pipelines, piping, valves, vessels, heat exchangers and many other pieces of equipment in facilities. There are many CRAs to select from, and they can be characterised by their resistance to specific environments.
Key environmental parameters influencing the corrosion properties of CRAs are:
• Temperature
• Chloride ion concentration
• Partial pressure CO2
• Partial pressure H2S
• Environment pH
• Presence or absence of Sulphur
Between them these parameters influence
• the stability of the passive film (initiation of pitting or general corrosion)
• ease of repassivation of initiated pits
• rates of dissolution of metal from pits
• the risk of Stress Corrosion Cracking (SCC) initiating and propagating
Key environmental parameters influencing the corrosion properties of CRAs are:
• Temperature
• Chloride ion concentration
• Partial pressure CO2
• Partial pressure H2S
• Environment pH
• Presence or absence of Sulphur
Between them these parameters influence
• the stability of the passive film (initiation of pitting or general corrosion)
• ease of repassivation of initiated pits
• rates of dissolution of metal from pits
• the risk of Stress Corrosion Cracking (SCC) initiating and propagating
Methods for Selecting CRAs
The selection of Corrosion Resistant Alloys, CRAs, for producing and transporting corrosive oil and gas can be a complex procedure and if improperly carried out can lead to mistakes in application and misunderstanding about the performance of a CRA in a specific service environment.
There are a variety of ways individuals and companies select CRAs for anticipated well and flowline conditions. Companies with large research facilities typically initiate a test program that involves simulating the particular part of the field environment under study (i.e. flowlines versus downhole). Then a group of alloys, based on information available, is selected that represents a possible range of alternatives. Rather than test all alloys all the time, it is more cost effective and less time consuming to test only a few CRAs that are likely candidates. This approach can easily require 1‑3 years to accomplish at considerable expense.
Another selection procedure is to review the literature for corrosion data that generally applies to the anticipated field conditions. This can result in elimination of those CRAs that are not good candidates and, thus, narrow the number of candidate alloys for testing. The selected CRAs are then tested under very specific conditions to fill gaps in literature data and/or field experience.
Care must be taken when using this approach because, for example, the corrosion resistance of many CRAs at one temperature is not necessarily indicative of their corrosion resistance at other temperatures. Likewise, changes in critical environmental components such as elemental sulphur can have a profound impact on the resistance to stress corrosion cracking (SCC), another important factor in alloy selection.
Other resources for materials selection are also available such as the 2003 ISO 15156 publication which was derived from the previous NACE 0175 publication for “sour service” and the EFC16 publication which incorporated the influence of environment pH on the suitability of materials for sour service. The ISO15156 standard covers different alloy types with separate tables for different applications; though field experience in many cases has shown that alloys will withstand more aggressive conditions. Corrigenda are published from time to time to update its contents and so it is important to obtain the latest version and corrigenda.
There are a variety of ways individuals and companies select CRAs for anticipated well and flowline conditions. Companies with large research facilities typically initiate a test program that involves simulating the particular part of the field environment under study (i.e. flowlines versus downhole). Then a group of alloys, based on information available, is selected that represents a possible range of alternatives. Rather than test all alloys all the time, it is more cost effective and less time consuming to test only a few CRAs that are likely candidates. This approach can easily require 1‑3 years to accomplish at considerable expense.
Another selection procedure is to review the literature for corrosion data that generally applies to the anticipated field conditions. This can result in elimination of those CRAs that are not good candidates and, thus, narrow the number of candidate alloys for testing. The selected CRAs are then tested under very specific conditions to fill gaps in literature data and/or field experience.
Care must be taken when using this approach because, for example, the corrosion resistance of many CRAs at one temperature is not necessarily indicative of their corrosion resistance at other temperatures. Likewise, changes in critical environmental components such as elemental sulphur can have a profound impact on the resistance to stress corrosion cracking (SCC), another important factor in alloy selection.
Other resources for materials selection are also available such as the 2003 ISO 15156 publication which was derived from the previous NACE 0175 publication for “sour service” and the EFC16 publication which incorporated the influence of environment pH on the suitability of materials for sour service. The ISO15156 standard covers different alloy types with separate tables for different applications; though field experience in many cases has shown that alloys will withstand more aggressive conditions. Corrigenda are published from time to time to update its contents and so it is important to obtain the latest version and corrigenda.
The quickest and least expensive alloy selection method is simply to review the literature, and existing or similar field data, and make the selection. This method can be quite unsatisfactory since certain critical factors or conditions will not be known and must be assumed. A greater chance for error exists in this selection approach, introducing a potential for failure of the CRA or use of a more expensive alloy than is required. It is advisable, if this method is used, to consult with someone who has a working knowledge of CRAs and their applications.
Finally, a CRA selection method that is not recommended but is often used is to select a CRA that is readily available or most economical, without regard to its corrosion resistance in the intended environment. Misapplication of CRAs is becoming more common for this reason and has resulted in corrosion and cracking problems of the inappropriately selected alloys.
However, it is recognized that before extensive efforts are made to make a final CRA selection for a specific application it is often desirable, if not necessary, to make preliminary selections of candidate CRAs to test in a simulated field environment or to perform an economic analysis to judge the cost effectiveness of several corrosion control alternatives (i.e. carbon steel plus inhibitors, CRAs, etc.). It is for these latter needs that these guideline diagrams are offered and should only be used in that fashion. More detailed testing and analysis is often required in order to make a final selection. Moreover, these diagrams are based almost entirely on laboratory data since they are often more conservative than field conditions and because laboratory data are more quantitative and frequently more accurate. The diagrams are based on corrosion rates for the alloys of less than or equal to 0.05 mm/y (2 mils/year) and resistance to sulphide stress cracking (SSC) and SCC. In this regard it should be noted that none of the diagrams indicate strength level. Generally, if NACE MR0175/ISO 15156 requirements are met, strength (and hardness) will not be an issue. However, it must always be borne in mind that increasing strength of an alloy will generally increase susceptibility to SSC and SCC.
Finally, a CRA selection method that is not recommended but is often used is to select a CRA that is readily available or most economical, without regard to its corrosion resistance in the intended environment. Misapplication of CRAs is becoming more common for this reason and has resulted in corrosion and cracking problems of the inappropriately selected alloys.
However, it is recognized that before extensive efforts are made to make a final CRA selection for a specific application it is often desirable, if not necessary, to make preliminary selections of candidate CRAs to test in a simulated field environment or to perform an economic analysis to judge the cost effectiveness of several corrosion control alternatives (i.e. carbon steel plus inhibitors, CRAs, etc.). It is for these latter needs that these guideline diagrams are offered and should only be used in that fashion. More detailed testing and analysis is often required in order to make a final selection. Moreover, these diagrams are based almost entirely on laboratory data since they are often more conservative than field conditions and because laboratory data are more quantitative and frequently more accurate. The diagrams are based on corrosion rates for the alloys of less than or equal to 0.05 mm/y (2 mils/year) and resistance to sulphide stress cracking (SSC) and SCC. In this regard it should be noted that none of the diagrams indicate strength level. Generally, if NACE MR0175/ISO 15156 requirements are met, strength (and hardness) will not be an issue. However, it must always be borne in mind that increasing strength of an alloy will generally increase susceptibility to SSC and SCC.
Technical Requirements
As before mentioned the lack of recognised standard is the major concern for procuring phase and subsequent manufacturing. Actually the only available standard is API 5CT which only covers grade 13 % Cr. steel mainly addressing mechanical and dimensional requirements. No standards are available for materials of Groups 2 to 4. For these alloys the purchaser need to develop tailored specifications; the alternative is to use the manufactures one’s. In this case the experience of the supplier should be the key point for its selection. The main points that should be addressed in the technical specification for some of the CRA materials are discussed.
Group-1 Martensitic-Martensitic/Ferritic Stainless Steel
The following features should be addressed in the technical specification:
- Chemical composition.
- Heat treatment
As before mentioned one of the 13 Cr.’s advantages over the most other CRA material is that its strength is obtained by austenize and tempering. Tubes are generally austenized at about 980 °C and because of its excellent hardenability, air cooled that resulted in fully martensitic structure. Tempering temperature is about 710 °C. NACE Standard MR- 01-75 requires double tempering for all martensitic stainless steels when used in sour environments, but there is no evidence that the double tempering improves the material resistance to H2S environments. Pipe manufacturers apply only one tempering.
- Microstructure checks
The only requirements to be inserted are related to delta ferrite content that shall not exceed 5% and microstructures
are required to have grain boundaries with no continuos precipitates.
are required to have grain boundaries with no continuos precipitates.
- Mechanical Properties
1) Yield and tensile strength
The most common yield strength range varies from 80 to 110 Ksi with a minimum tensile of 90 Ksi. Depending on the service condition and supplier manufacturing experience, a frequency of one tensile test for each lot of 100 or 200 tubes is reasonable.
The most common yield strength range varies from 80 to 110 Ksi with a minimum tensile of 90 Ksi. Depending on the service condition and supplier manufacturing experience, a frequency of one tensile test for each lot of 100 or 200 tubes is reasonable.
2) Hardness
The NACE MR-01-75 limit of 22 HRC for the 80 Ksi minimum yield strength, is a difficult task for type 420 due to its high yield-to-tensile-ratio. As suggested by API 5 CT, a more realistic value is 23 HRC. For upset pipes it is a good practice to limit the difference in hardness between the readings in the quadrants. Surface hardness tests with a portable Rockwell type tester is not recommended due to the unreliability of the measurement.
The NACE MR-01-75 limit of 22 HRC for the 80 Ksi minimum yield strength, is a difficult task for type 420 due to its high yield-to-tensile-ratio. As suggested by API 5 CT, a more realistic value is 23 HRC. For upset pipes it is a good practice to limit the difference in hardness between the readings in the quadrants. Surface hardness tests with a portable Rockwell type tester is not recommended due to the unreliability of the measurement.
- Impact Properties
The impact properties at low temperatures should be determined. The suggested values of the minimum impact adsorbed energy. Suggested test temperature is -10°C. In case the minimum service temperature is less than -10 °C, the test temperature should be agreed with the manufacturer.
Group 2 Duplex Stainless Steel
Duplex stainless steel offer several advantage over the martensitic alloy. The duplex grades are highly resistant to chloride stress corrosion cracking, have a good crevice and pitting corrosion resistance. They are available in a wide yield strength range from 65 Ksi up to 140 ksi. Actually there is no standard that cover such materials, therefore the following features shall be carefully evaluated:
- Chemical composition.
In general it is recommended to be at the high end of the range for chromium an molybdenum, while the sulphur
content should be kept as low as possible.
content should be kept as low as possible.
- Heat treatment
According to the final size, manufacturing process pipes may undergo a solution annealing treatment, after heat extrusion or between intermediate and final cold working phase The scope of the heat treatment is to obtain the best microstructure while maintaining carbides is solid solution and relieve all stresses. For a best stabilisation of ferritic and austenitic phases the material needs to receive a direct quenching after heat treatment.
- Hardness
The NACE MR-01-75 limit of 28 HRC for the solution annealed condition is acceptable. The limit of 36 HRC for the high-strength cold worked condition is not achievable for the 125/140 grades. A more realistic value is 37/38 HRC respectively.
- Microstructure checks
The microstructure shall have a ferritic-austenitic structure. The microstructure is required to have grain boundaries with no continuos precipitates. Intermetallic phases, nitrides and carbides shall not exceed 1,0% all together. Sigma phase shall not exceed 0,5%. The ferrite volume fraction shall be in the range 40% to 60% for alloys with a minimum PRE<40 (duplex) and in the range 35% to 55% for alloys with a minimum PRE³ 40 (super duplex).
- Impact Properties
The impact properties at low temperatures should be determined. The suggested values of the minimum impact adsorbed energy. Suggested test temperature is -10°C. In case the minimum service temperature is less than -10 °C, the test temperature should be agreed with the manufacturer. As we move to Group 3 and 4 alloys, the amount of alloying increases up to eight times more nickel and three times more molybdenum while maintaining about the same chromium content. Group 3 and 4 alloys are chosen for improved corrosion resistance to H2S, CO2 and chlorides. The chemistry of these alloy are very important as far as the microstructure check to evaluate the absence of carbide precipitates at grain boundaries that can compromise the corrosion resistance. Intermetallic phases, nitrides and carbides should not exceed 1,0%. Sigma phase should not exceed 0,5%.
In addition to chemical and metallurgical evaluations, corrosion testing are also recommended to verify that the materials will met the expected performances. The specification should include accelerated corrosion tests because testing in standard condition would take several months. Slow Strain Rate Tensile Test (SSRT) is a test that can usually be requested because of its short duration. The standard test condition are 300 °F, 100 psi H2S partial pressure at ambient pressure and temperature, 25 percent NaCl brine, and 0.5 percent acetic acid.
In addition to chemical and metallurgical evaluations, corrosion testing are also recommended to verify that the materials will met the expected performances. The specification should include accelerated corrosion tests because testing in standard condition would take several months. Slow Strain Rate Tensile Test (SSRT) is a test that can usually be requested because of its short duration. The standard test condition are 300 °F, 100 psi H2S partial pressure at ambient pressure and temperature, 25 percent NaCl brine, and 0.5 percent acetic acid.
Manufacture Process
For manufacturing the CRA alloys there are essentially two processes. Group 1 comprises martensitic and martensicferritic stainless steel, they are manufactured in a manner similar to carbon steel. The alloy is melted in an electric furnace then it is cast into ingots. The ingot is forged to form a billet that is heated to a suitable forging temperature, pierced and hot rolled to form a pipe. In order to achieve the mechanical properties, the pipe then is quenched and tempered. Groups 2, 3 and 4 alloys, such as duplex stainless steel and austenitic-nickel-base alloys, are fabricated in different manner. After melting the material can mold to form an ingot or it can be continuously cast. The ingot is then forged into billets that are extruded by the back-extrusion press. In the majority of cases these grades are required in relatively high strengths which require the alloys to be cold worked. This cold work is performed on either cold draw benches or in a cold pilger mill. Several passes on the draw bench may be necessary to achieve the correct strength while in general only a sizing pass and the finishing pass are requested on the pilger mill.
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